In playtests, how does she not go OoM? Is she bound to PoM+Manaflow? Do you envision her building Tear? Tear+ER? Doran's Ring, Tear, ER?
Have you considered making the W bounces apply on-hits? Or, at least, proc spell effects? It's hilarious to me that a level two Sivir with a Jungle item just dies killing Raptors. Not that Jungle Sivir is on your radar, but it's one of only a handful of abilities that don't activate Tooth and Nail when it hits enemies.
The intention is for her to be similarly "bound" to PoM/ER as Jinx and Lucian. This is an area we are actively tuning however so those numbers may change.
Yes considered, decided against it. On-hit items are a scary system that may cannibalize her builds (titanic!?) and the spell effects part is spooky for things like Aery, comet, and scorch in lane. Overall, current functionality is more than fine.